There are two basic objectives of the studies proposed herein: 1) to evaluate several mouse models designed to measure the ability of hemopoietic stromal tissue to support hemopoiesis; and 2) to evaluate the type, extent, and duration of damage inflicted on murine marrow and spleen hemopoietic stromal tissue by chemotherapeutic agents and radiation. Four murine models to measure the integrity, or hemopoietic support capacity, of hemopoietic stromal tissues (or microenvironment) have been designed: 1) The first, will evaluate the ability of the spleens of whole-body lethally irradiated mice to support clonogenic pluripotent stem cell (CFU-S) proliferation; 2) The second, will study the hemopoietic compensatory capacity of the spleens of 89Sr marrow-ablated mice; 3) The third, will determine the amounts of hemopoiesis contained in subcutaneously implanted femurs and spleens; and 4) The fourth, will measure hemopoietic recuperation in femurs and spleens after lethal whole-body irradiation and rescue with normal marrow cells. Wherever applicable, pluripotent precursor cells (CFU-S), granuloid precursor cells (CFU-C) and erythroid precursor cells (CFU-E) will be measured. 99mTc uptake of femurs and spleens and blood flow in implanted organs will also be evaluated in selected studies.